QDataStream有时使用32位浮点数，有时使用40位浮点数 [英] QDataStream uses sometimes 32 bit and sometimes 40 bit floats

问题描述

我正在编写一个应将浮点数组写入WAVE文件的应用程序.我为此使用了QDataStream，但这导致了我无法解释的非常不可能的输出.似乎QDataStream有时选择32位浮点数，有时选择40位浮点数.由于必须遵守严格的格式，因此会弄乱整个输出文件.

I am writing an application that is supposed to write an array of floats to a WAVE file. I am using a QDataStream for this, but this results in a very improbable output that I can't explain. It seems like the QDataStream sometimes chooses 32 bit floats and sometimes 40 bit floats. This messes up the entire output file, since it has to obey a strict format.

我的代码大致如下:

float* array;
unsigned int nSamples;

void saveWAV(const QString& fileName) const 
{
    QFile outFile(fileName);
    if (outFile.open(QIODevice::WriteOnly | QIODevice::Text)) 
    {
        QDataStream dataStream(&outFile);
        dataStream.setByteOrder(QDataStream::LittleEndian);
        dataStream.setFloatingPointPrecision(QDataStream::SinglePrecision);

        // ... do all the WAV file header stuff ...

        for(int ii = 0; ii < nSamples; ++ii) 
            dataStream << array[ii];
    }
}

我认为没有任何理由使这段代码有如此副作用.因此，我举了一个最小的例子来了解正在发生的事情.我将for循环替换为此:

I can think of no reason of how this code could have such a side-effect. So I made a minimal example to find out what was going on. I replaced the for-loop by this:

float temp1 = 1.63006e-33f;
float temp2 = 1.55949e-32f;

dataStream << temp1;
dataStream << temp1;
dataStream << temp2;
dataStream << temp1;
dataStream << temp2;

然后我使用Matlab打开输出文件，并查看写入文件的字节.这些是:

Then I opened the output file using Matlab and had a look at the bytes written the file. Those were:

8b 6b 07 09      // this is indeed 1.63006e-33f (notice it's Little Endian)
8b 6b 07 09 
5b f2 a1 0d 0a    // I don't know what this is but it's a byte to long
8b 6b 07 09 
5b f2 a1 0d 0a

我随意选择了这些值，它们恰好具有这种作用.一些值导出为4字节，另一些导出为5字节数字.有谁知道这可能是什么原因?

I chose the values pretty arbitrarily, they just happened to have this effect. Some values are exported as 4-byte and other ones as 5-byte numbers. Does anyone have any idea what may be the cause of this?

修改: 检查两个浮子的大小时，它们似乎确实是4 char长，但是:

When checking the size of both floats, they do seem to be 4 chars long, though:

    qDebug() << sizeof(temp1); // prints '4'
    qDebug() << sizeof(temp2); // prints '4'

推荐答案

答案就在于输出文件的打开:QIODevice::Text标志应该被省略了，因为它是一个二进制文件.如果包含文本标志，则在每个0a之前插入一个0d字符.因此，每个包含0a字符的浮点数似乎都会因此而char更长.

The answer lies in the opening of the output file: the QIODevice::Text flag should have been left out, since it is a binary file. If the text-flag is included, a 0d character is inserted before each 0a. So each float that contains an 0a character seems a char longer because of this.

此答案的所有学分归于以下给出的答案: 浮动长度在32到40位之间变化

All credits for this answer go to the answers given in: Length of float changes between 32 and 40 bit

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